1. Field of the Invention
The present invention relates to a radio transmitter-receiver system for increasing or decreasing communication channels, more particularly to a radio transmitter-receiver system for increasing or decreasing communication channels by varying a frequency of an intermediate frequency (IF) local oscillator in a heterodyne type radio transmitter-receiver instead of using a duplexer for a radio frequency (RF) signal.
2. Description of the Related Art
In a communication system wherein the communication network is constituted by lines provided among many node stations, and back-up lines are provided between each station, for example, when lines are provided between stations A, B, C, and D, if there is a breakdown in the line between stations A and D, communication between the stations A and D is established through a line between stations A and C and a line between stations C and D. A different frequency is usually applied to each line, and each line has a transmission capacity of, for example, 1200 channels per one frequency.
In addition, a standby radio transmitter-receiver is usually provided to continue communications in the event of a malfunction of the working radio transmitter-receiver, and thus a station generally comprises both a working and a standby radio transmitter-receiver.
Conventionally, a heterodyne type transmitter-receiver is used. The transmitter receives a baseband input and comprises a low-pass filter, three amplifiers, a modulator, an intermediate frequency oscillator, a radio frequency oscillator, two mixers, and two band-pass filters. An RF (radio frequency) output is output from one of the amplifiers. The receiver receives an RF input and comprises a radio frequency oscillator, an intermediate frequency oscillator, two mixers, two band-pass filters, three amplifiers, a demodulator, and a low-pass filter. A baseband output is output from one of the amplifiers.
In such a communication network, if a line failure occurs, such as a deterioration in transmission characteristics caused by heavy rain, an obstruction by microwave shielding matter, damage to an antenna, or the like, communication by even a normal radio transmitter-receiver becomes difficult or impossible. If the channel capacity of the back-up line is provided with a sufficient margin, the failed line can be by-passed and eventually replaced. However, if the margin is insufficient, a back-up can be provided for only a short time, but the replacement of the failed line may take a long time, and thus there is a drop in capacity during the intervening period.
The present invention intends to solve these problems by changing the constitution of the radio transmitter-receiver, and thus making it possible to increase or decrease the number of channels in the line.